Does a signer need an account?

Not necessarily, however this would depend on the signature policy selected while composing the ePak for signature and the system configuration. But irrespective of the policy, your signer will need to have an email address it can send the ePak (document) to.

By default, MSB does not require a user to have an account except for a 21 CFR signature policy however the system can be configured to make account registration mandatory before letting a user access any ePak sent to them for signature/review.